Studies on Alu repetitive sequences have further established that these elements are maintained in a transcriptionally silent state by complex and potentially redundant mechanisms. Concurrent investigation of the retinoblastoma tumor suppressor gene product (pRb) led to discovery that this protein can modulate exit from mitosis as well as G1 to S phase transit. Together, Alu and Rb studies prompted formulation of a model in which heterochromatin maintenance serves as an unconventional and previously unappreciated cell cycle control mechanism that may be compromised in senescent cells.